System and method of conducting particle monitoring using low cost particle sensors

ABSTRACT

There is disclosed a field calibratable particle sensor solution in a low-cost, very compact form factor. This makes a low-cost sensor more accurate for low-concentration pollution measurements and decreases the cost of pollution measurement systems having a wide geographic coverage. In a related embodiment, the invention illustrates a method and system to remotely and automatically calibrate one or more of the low cost sensors disclosed herein as well as other commercially available sensors (such as optical particle counters, photometers etc.) against a reference instrument (such as a beta attenuation monitor) which may or may not be physically located in the same place as the individual sensors. The method may require minimum (or no) user interaction and the calibration period is adjustable periodically.

CLAIM OF PRIORITY

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 15/661,999, filed on Jul. 27, 2017, now U.S. Pat.No. 10,041,862, which is a divisional of and claims priority to U.S.patent application Ser. No. 14/586,122, filed on Dec. 30, 2014, now U.S.Pat. No. 9,726,579, which claims priority to and the benefit of U.S.Provisional Patent Application No. 62/086,414, filed Dec. 2, 2014 andtitled “SYSTEM AND METHOD OF CONDUCTING PARTICLE MONITORING USING LOWCOST PARTICLE SENSORS” all of which applications are incorporated hereinby reference in their entireties.

FIELD AND BACKGROUND OF THE INVENTION

The invention generally relates to the detection of particles, and morespecifically to the measurement of dust particle concentrations and sizedistributions.

When inhaled, aerosol particles can deposit on the respiratory track andcause adverse health effects. Hence, industry and government haverecognized the importance of measuring and monitoring aerosolconcentrations in the environment or workplace so that proper measurescan be taken to reduce potential health risks. Pertinent monitoringapplications include but are not limited to commercial building or homeair quality monitoring, industrial/occupational hygiene surveys, outdoorambient/site perimeter monitoring for dust control operations, andengine emission studies. Some industrial processes require knowledge ofthe particulates in the environment, including environments having asparse population of particles (e.g., semiconductor clean roommanufacturing or pharmaceutical drug processing) as well as environmentshaving an extensive presence of particle populations (e.g., dry powdermanufacturing processes). In 1987, the United States EnvironmentalProtection Agency (EPA) revised the National Ambient Air QualityStandards (NAAQS) and started to use mass of particles with aerodynamicdiameters less than approximately 10 μm (hereinafter “the PM10”) as theparticulate matter (PM) pollution index. The PM10 is an index of the PMthat can enter the thorax and cause or exacerbate lower respiratorytract diseases, such as chronic bronchitis, asthma, pneumonia, lungcancer, and emphysema. It was later determined that PM concentrations inthe air, as indexed by the mass of particles with aerodynamic diametersless than approximately 2.5 μm (“PM2.5”) was more closely associatedwith the annual mortality rates than with the coarser PM10. In 1997, inits next revision of the NAAQS, the EPA promulgated regulations onPM2.5.

The American Conference of Governmental Industrial Hygienists (ACGIH)has also established sampling conventions of respiratory, thoracic andinhalable aerosols, defined as particles having aerodynamic diameters ofless than 4 μm, 10 μm, and 100 μm respectively. Inhalable particles arethose capable of entering through the human nose and/or mouth duringbreathing. Thoracic particles are the inhaled particles that maypenetrate to the lung below the larynx. Respiratory particles are theinhaled particles that may penetrate to the alveolar region of the lung.A discussion of the various sampling conventions are found at NationalPrimary and Secondary Ambient Air Quality Standards, 40 Code of USFederal Regulation, Chapter 1, Part 50 (1997) and Vincent, J. H.,Particle Size-Selective Sampling for Particulate Air ContaminantsCincinnati, ACGIH (1999), both of which are hereby incorporated byreference except for explicit definitions contained therein.

While the aforementioned standards and conventions are based on theaerodynamic diameters of particles, it is understood that sizesegregated mass concentration groupings (e.g., PM1, PM2.5, PM10,respirable, thoracic and inhalable) may be based on the optical particlediameters instead of the aerodynamic diameters for purposes of theinstant application. That is, PM2.5 (for example) may approximateparticles having an aerodynamic diameter of less than approximately 2.5μm or particles having an optical diameter of less than approximately2.5 μm. Particle mass measurements can be achieved in real time using aphotometer if the aerosol is primarily a fine aerosol (approximatelybetween 0.1- and 4-μm). The photometer is a device that produces anelectrical signal that varies with the intensity of scattered lightreceived from a particle or an ensemble of particles in theinterrogation volume region. The photometric signal can be approximatelycorrelated to particle mass. The photometer may also be sensitive to awide dynamic range of particle concentration. For example, the TSI Model8520 DUSTTRAK photometer measures a particle mass concentration range of0.001- to 100-mg/m³ over the particle size range of 0.1- to 10-μm.

Government regulations exist in many countries that require monitoringof various pollution parameters. Instrumentation for measuring theseparameters according to regulations tends to be expensive, on the orderof $15K-$100K, and the operational costs can be very expensive as well.Due to the cost, there are few instruments in a given geographical areato indicate air quality. Since pollution sources can be localized, thereis a great deal of interest in measuring at many points within ageographical area. Sensors for many distributed measurements must beinexpensive but still give a fairly accurate measurement compared tohigher precision instruments. There are low-cost techniques for givingan indicative measurement of particulate mass but they are very limitedin accuracy. The most common are photometric sensors that measure anensemble of light scattered from a light source (usually a laser, LED orother source of intense light) and detected by a photodetector (usuallya photodiode or other sensitive light detector). The most commonlow-cost option is an LED light source and photodiode detector. Thesetypes of sensors incorporate electronics that convert the signal fromscattered light to an electrical signal that can be processed to give auser an indication of particle mass on a display.

In other instances, gravimetric sampling is used which consists ofcollecting particles, usually over a long period of time such as 24hours, on a pre-weighed filter. The weight of the filter and particlesare then measured. The difference in weight between the filter beforeand after sampling provides the weight of the particles in a givenperiod of time for a given sampling volume. Some of the advantages offilter sampling include: 1) it is relatively inexpensive to set up andimplement and then interpret the data; and 2) the concepts ofmeasurement based on first principles are easier to grasp and share withother interested parties. Other the other hand, gravimetric filteringdoes have a higher total cost of measurement which includes a great dealof labor cost making it very expensive, trying to measure low particleconcentrations can be very difficult since the measurement is adifference in two weight conditions (unused and used filter), poor timeresolution, the process is labor intensive and operator error is apossibility, and particles may evaporate before they are weighed.

Photometer sensors tend to require frequent calibration of zero and spandue to sensor drift with temperature, humidity or other outside factors.Some disadvantages of various photometers are: (1) only the total massis measured (no particle size segregated mass information is provided);(2) the photometric signal is dependent on particle properties such assize, shape and refractive index, thus requiring different calibrationfactors for different aerosols; (3) photometers are typically moresensitive to particles having diameters close to the wavelength of thelight source, with a precipitous drop off in signal per unit mass forparticles outside of this size range; and (4) photometers canunderestimate particulate mass if the sampled aerosol contains particleslarger than 4 μm.

One instrument that measures particle size dependent numberconcentrations in real time is the optical particle counter (OPC). In anOPC, individual particles pass through an interrogation volume that isilluminated by a light beam. The light scattered by each particle iscollected on to a detector to generate an electrical pulse. From thepulse height and/or pulse area (i.e. the intensity of the scatteredradiation) one can infer the particle size based on prior calibration.Because the size inferred from the OPC depends on the particle opticalproperties, the inferred parameter is often referred to as the “opticalequivalent particle size.” Some advantages of the OPC are: (1) particlesmay be counted with high accuracy for low particle concentrations; (2)favorable signal to noise ratios for particle sizes greater than 1 μm;and (3) low cost. However, the inferred particle optical size may not bethe same as the actual or geometric particle size because thedetermination depends on the particle shape and refractive indexassumptions. Additional errors may arise when converting the particlesize distribution to a mass concentration if the particle density isincorrectly assumed. Furthermore, OPCs typically underestimate particleconcentration when multiple particles are present in the interrogationvolume region (a condition often referred to as “coincidence error”).Accordingly, OPCs are typically only used in relatively cleanenvironments. An example is the TSI Model 9306 OPC, which counts 95% ofparticles at a number concentration of approximately 200 particles/cm³or mass concentrations less than 1-mg/m³. The counting efficiency of theModel 9306 drops quickly as concentration increases above these limits.

In summary, filter sampling provides first principle mass measurement,but has poor time resolution and it does not provide particle sizeinformation. Obtaining size segregated mass concentration measurementsmay require the procurement and maintenance of multiple instruments. Thephotometer measures a wide particle concentration range, but it does notprovide particle size information and may be relatively insensitive toparticles having diameters greater than approximately 4-μm. Aninstrument and system that can provide size segregated particle massconcentrations information in real time and over a wide range of massconcentrations and a wide geographical area at a competitive cost andsimplifies remote calibration of would be a welcome improvement.

SUMMARY OF THE INVENTION

The various embodiments of the invention provide a field calibratableparticle sensor solution in a low-cost, very compact form factor. Thismakes a low-cost sensor more accurate for low-concentration pollutionmeasurements and decreases the cost of pollution measurement systemshaving a wide geographic coverage.

In a related embodiment, the invention illustrates a method and systemto remotely and automatically calibrate one or more of the low costsensors disclosed herein as well as other commercially available sensors(such as optical particle counters, photometers etc.) against areference instrument (such as a BAM—beta attenuation monitor) which mayor may not be physically located in the same place as the individualsensors. The method may require minimum (or no) user interaction and thecalibration period is adjustable periodically (hourly, daily, weekly,etc. . . . ). Optical sensor calibration is used to explain this exampleembodiment of the invention.

Optical sensors such as optical particle counters and photometers arecommonly used in the aerosol field. These sensors measure the lightscattered or attenuated by the particles. The light signal dependsstrongly on the aerosol properties namely refractive indices andmorphology. The effect of the refractive indices and morphology can betaken into account by either performing theoretical scattering modelingif aerosol optical properties are known or calibrating the instrumentsagainst a measurement reference. If the optical sensors are calibratedagainst a non-optical reference instrument, then there is an additionalbenefit in that using this method, optical signals measured by thesesensors would automatically be converted to another useful aerosolproperty of interest. For instance, if the optical sensors arecalibrated against a reference mass measurement instrument, instead ofshowing optical signals, these sensors can then provide aerosol massinformation. If the reference instrument is an aerodynamic sizemeasurement instrument, these optical sensors can then be used tomeasure aerosol aerodynamic size after calibration. Ideally, opticalsensors should be re-calibrated every time the composition of theaerosol changes. In practice, however, calibration is usually only doneinfrequently because the reference instruments are typically onlyavailable in the laboratories or at certain fixed locations due to theirlarge size and high cost. Further, the calibration process is typicallylabor intensive and expensive so it is not practical to performcalibrations frequently in the field. One example embodiment of theinvention provides a method and system to calibrate these remotelylocated sensors easily and without the requirement of having a referenceinstrument physically located in the same place as the sensor(s).

In one example embodiment, there is provided a sensor assembly forsensing low concentrations of particulate matter that includes a housinghaving a front and rear portions wherein the rear portion includes anair channel configured to direct a sampled particle aerosol from therear portion through to the front portion. The assembly further includesa particle sensor device having a front and rear surface and a flowchannel therethrough that spans from the rear surface to the frontsurface, the rear surface of the particle sensor device disposed overthe housing air channel such that at least a portion of the sampledparticle aerosol flows into the flow channel of the particle sensordevice. In addition, a microblower member interposed between the airchannel of the housing and the rear surface of the particle sensordevice, the microblower adapted to periodically push air through theflow channel of the particle sensor device so as to zero the particlesensor before a subsequent reading; and a filter element is disposedadjacent the microblower and the air channel. In a related embodiment,the sensor assembly forms part of an air quality monitoring systemhaving at least one sensor assembly that may be field calibrated, atleast one sensor assembly communicatively coupled to a mobile device,the mobile device in communication with a server adapted to receive acalibration factor, wherein the calibration factor is generated fromdata received from a reference instrument and from a transfer standardmodule, one of which is remotely located from the at least one sensor tobe calibrated.

In another example embodiment, there is provided a particle sensorassembly calibration system that includes at least one remotely locatedparticle sensor assembly configured to be calibrated in association withat least one reference instrument, wherein the at least one referenceinstrument is configured to generate a calibration factor. In addition,a mobile device configured to communicate with the at least onereference instrument and with the at least one remotely located particlesensor assembly.

In yet another example embodiment, there is provided a particle sensorassembly calibration system that includes at least one remotely locatedparticle sensor assembly configured to be calibrated in association withat least one reference instrument, wherein the at least one referenceinstrument is configured to generate a calibration factor. In addition,a control device configured to communicate with the at least onereference instrument and with the at least one remotely located particlesensor assembly.

The novel features of the various embodiments the invention itself, bothas to its construction and its method of operation, together withadditional advantages thereof, will be best understood from thefollowing description of specific embodiments when read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other important advantages of the invention will be apparent from thefollowing detailed description of the invention taken in connection withthe accompanying drawings in which:

FIGS. 1A-1B are exploded and inset views, respectively, of a low costparticle sensor assembly in accordance with an example embodiment of theinvention;

FIGS. 2A-2B are side cutaway and 3-D views of the particle sensorassembly as taught herein and the direction of flow of the aerosolsample being taken in accordance with an example embodiment of theinvention;

FIGS. 3A-3D are views of a dust sensor device used in the particlesensor assembly in accordance with an example embodiment of theinvention;

FIG. 4 is a graph illustrating the particle sensor assembly performanceas taught herein in various conditions;

FIG. 5 is a top level system diagram of particle sensors that arecalibrated remotely in accordance with an example embodiment of theinvention;

FIG. 6 illustrates a system and method of calibrating particle sensorsusing an internet server in accordance with an example embodiment of theinvention; and

FIG. 7 illustrates a system and method of calibrating particle sensorsusing an internet server and a transfer standard in accordance with anexample embodiment of the invention

DETAILED DESCRIPTION OF THE INVENTION

Following are more detailed descriptions of various related conceptsrelated to, and embodiments of, methods and apparatus according to thepresent disclosure. It should be appreciated that various aspects of thesubject matter introduced above and discussed in greater detail belowmay be implemented in any of numerous ways, as the subject matter is notlimited to any particular manner of implementation. Examples of specificimplementations and applications are provided primarily for illustrativepurposes.

In one example embodiment, there is disclosed an inexpensive and verycompact device that addresses the requirement of frequent zerocalibration of a low-cost sensor. The low-cost particle sensorincorporates a unique and compact piezoelectric microblower and filtermaterial to periodically pass filtered air through a photometric sensorto provide more accurate low-concentration measurements. In this exampleembodiment, the piezoelectric microblower has an advantage over otherair movers in that it can provide enough pressure head across a filterto drive a significant flow (about 1 LPM) to provide clean air to a dustor particle sensor component in a very small package. This particlesensor assembly has application in indoor and outdoor air measurementsystems of PM 2.5 or other air pollution measurements. The specificdesign incorporates several design features that adapt a microblower ormicropump of this type to the application. In a related embodiment, sucha low cost particle sensor facilitates implementation of an overallparticle monitoring system that spans over a large geographical area dueto improved system cost for a user.

Referring now to the figures, and in particular FIGS. 1A-3, there isillustrated in FIGS. 1A-1B exploded and inset views, respectively, of alow cost particle sensor assembly 100 in accordance with an exampleembodiment of the invention. In this example embodiment, particle sensorassembly 100 includes a housing 102 that supports a circuit board 103,an aperture 104 for sample air flow, a dust sensor 110, a piezoelectricmicroblower 130 adapted to fit into aperture 104, a filter media 140 forthe rear of microblower 130 and a retainer clip 150 that holds all ofthe components together. Inset view of FIG. 1B illustrates the variouscomponents above all assembled. FIG. 2B illustrates a 3-D view ofparticle sensor assembly 100 fully assembled. In this embodiment,microblower 130 is as described in detail in U.S. Pat. No. 8,678,787,which is incorporated by reference herein in its entirety. In a relatedembodiment, blower 130 is configured from other designs and isconfigurable to be a micro pump, as described in U.S. Pat. No.8,066,494, which is incorporated by reference herein in its entirety.

Referring now to FIG. 2A, there is shown a cutaway side view of particlesensor assembly 100 as taught herein illustrating a flow channel 105 inhousing 102 in which sampled air travels in the direction of flow 106and moves through microblower 130 and cylindrical channel 114A of dustsensor 110. During operation, air to be sampled flows through flowchannel 105 along a wall of housing 102 and then is directed throughcylindrical channel 114A in which a light is directed perpendicular tothe flow through channel 114A. In this example embodiment, the filterelement is on the outside of the housing such that the particle ladenaerosol flow goes through the filter, then through the microblower andthen through the dust sensor. In this example embodiment, blower or pump130 uses a piezoelectric crystal that has a very compact size and iseasily driven by electronics. Piezoelectric microblower 130 providessignificant flow (about 1 lpm) to provide clean air to dust or particlesensor 110 in a very small package. The design allows for easyinstallation and replacement of filter media 140. It also diffuses thehigh-velocity output of the pump so it can clean out a larger sensorspace in a short time. The pump routes sample air flow without the useof tubing or O-rings. FIG. 4 is a graph illustrating performance of theparticle sensor assembly 100 as taught herein in various conditions,where the detection chamber is purged with clean air (Sharp 4 with Zeropump) to improve the accuracy of the dust sensor at very lowconcentrations.

Referring now to FIGS. 3A-3D, there are a sectional, front, bottom, andrear views, respectively, of optoelectronic dust sensor 110 used insensor assembly 100. An opening 113 a is formed at front panel 113 ofmain body housing 112, and flow channel or passage hole 114 a is formedat back panel 114 of main body housing 112, with the flow channel ordust passage route permitting passage of dust and/or smoke or smog beingprovided between passage hole 114 a of back panel 114 and opening 113 aof front panel 113. Passage hole 114 a is for introducing dust and/orsmoke to the dust passage route. Opening 113 a, being for dischargingdust and/or smoke from the dust passage route, is sufficiently largerthan passage hole 114 a. In this example embodiment, microblower 130 islocated at or near hole 114 a of sensor 110 so as to push air throughsensor 110 to clean and zero out the device by periodically activatingthe microblower. The microblower may be activated for a few minutes perhour or per week for this purpose. In one example embodiment the flow,although temporary, is a continuous, generally pulse-free air flow, andit is continuously on for a period of 2-5 minutes in one exampleembodiment. Further, it can be periodic, such as once per minute or onceper week.

Furthermore, a light-emitting unit 115 and a light-receiving unit 116are respectively arranged so as to be directed toward the dust orparticle passage route. In this example embodiment, a plurality ofoptical baffles 117 are arranged in distributed fashion as appropriate,preventing light from light-emitting unit 115 from being directlyincident on light-receiving unit 116 and forming optical isolationregion(s) 118. In this example embodiment, light-emitting unit 115 isequipped with light-emitting element 115 a, lens 115 b, and slit 115 c;light from light-emitting element 115 a being collimated by lens 115 b.The cross-section of the collimated light beam is narrowed and/or shapedby slit 115 c, and this thereafter exits therefrom such that it isdirected at the dust passage route. Light-receiving unit 116 is equippedwith light-receiving element 116 a, lens 116 b, and slit 116 c; withlight from the dust passage route being condensed onto light-receivingelement 116 a by way of slit 116 c and lens 116 b.

In this example embodiment, lens 115 b and slit 115 c of light-emittingunit 15 cause the light from light-emitting element 115 a to beconcentrated in which light from light-emitting element 115 a could bedispersed and reflected within main body housing 112, thereby causingunwanted light to be incident on light-receiving unit 116. Furthermore,lens 116 b and slit 116 c of light-receiving unit 116 cause light whichis reflected by dust and/or smoke in the dust passage route to bereceived at light-receiving element 116 a, thereby preventing situationsin which unwanted light reflected within main body housing 112 isreceived at light-receiving element 116 a. In the event that there is nodust or smoke passing through the dust passage route, then light fromlight-emitting unit 115 will pass through the dust passage route andreach optical isolation region 18, hence at this optoelectronic dustsensor 110 will sense that the amount of light received atlight-receiving unit 116 will be extremely small.

Conversely, in the event that there is dust and/or smoke or smog passingthrough the dust passage route, because a portion of the light fromlight-emitting unit 115 will be reflected by the dust and/or smoke inthe dust passage route and will be incident on light-receiving unit 116,the amount of light received at light-receiving unit 116 will increase.Accordingly, presence and/or absence of dust and/or smoke passingthrough the dust passage route may be detected based on variation inreceived-light output at light-receiving element 116 a oflight-receiving unit 116. In addition, the concentration of dust and/orsmoke passing through the dust passage route may be detected based onthe received-light output level at light-receiving element 116 a. Afurther description of the operation and the electronic circuits formingthe optoelectronic sensor 110 is described in U.S. Pat. No. 7,038,189,which is incorporated herein by reference in its entirety.

Referring now to FIGS. 5-7, and in particular FIG. 5, there isillustrated a high level view of system and method 200 for remotelycalibrating one or more particle sensors with one or more referenceinstruments that are not necessarily co-located with each other inaccordance with an example embodiment of the invention. In particular,system 200 includes a plurality of particle sensors 210A-210D that aredeployed in the field along with one or more reference instrument(s) 220and a transfer standard module 230 that is used to facilitate the remotesensor calibration process. To ensure good calibration, transferstandard module 230 is preferred to be (but not necessarily limited to):(1) the same type of particle sensor which is under calibration or (2) asensor that provides good correlation to the particle sensors undercalibration. The readings of reference instrument 220 and transferstandard module 230 are first made available on the Internet or someother communications network. This can be done in a number of waysincluding uploading the data to a web site, sending the data via a shortmessage (for instance, social media service Twitter) or via a text. Asoftware module or firmware then downloads to reference instrument 220and transfer standard module 230 readings or data from the Internet to acalibration factor module 240. A calculation or processing of suchdata/readings is then performed at a calibration factor module 240 todetermine the calibration factor to be used on the remote particlesensors. Since different calibration factors may be necessary atdifferent locations or cities, the calibration factor specified to acertain location/city could be determined by using reference instrument220 and transfer standard module 230 stationed in that particularlocation/city. Then a lookup table is generated and it consists ofinformation about calibration factors at various locations or cities.This lookup table is then uploaded to a server 250. The content of thislookup table may refresh at certain periods such as hourly, daily,weekly, etc.

Once the lookup table is generated and uploaded to server 250, there aretwo ways to calibrate the sensors in the field. One way is to use amobile device 260 to download the lookup table from the internet andthen transfer or transmit the calibration factor to particle sensor 210Abased on the location information stored in the sensor. Thecommunication between mobile device 260 and sensor 210A is either wiredor wireless communication 270. If the calibration factor is notavailable at this location, users are able to input a custom calibrationfactor, use a value previously stored in the particle sensor, or use aninterpolated value based on the location of the particle sensor to becalibrated and its nearest reference instruments and transfer standards.The previous stored value could also be the factory-calibrated value.Since the signal from mobile device 260 may have very limitedtransmission range, it is possible only one particle sensor could becalibrated at a time.

In a related embodiment, in order to calibrate multiple particle sensorsat a time, a control device 280 and a broadcaster module 290 are usedand integrated into the calibration system. Control device 280 could bea computer or server while broadcaster module 290 could be a Wi-Firouter, Bluetooth device or a radio frequency broadcaster. Controldevice 280 downloads the lookup table from the internet or server 250,determines the calibration factor based on the physical location of theparticle sensor, and then sends the calibration factor to all of theparticle sensors 210B-210D via the broadcaster. The frequency ofcalibration of the sensors is configurable by control device 280.

Referring now to FIG. 6 there is illustrated a system 300 and method ofcalibrating particle sensors 310A-310D without using an internet serverin accordance with an example embodiment of the invention. In thisembodiment, the calibration factor is determined directly by mobiledevice 360 or control device 380 (as shown in FIG. 6). Hence, aninternet server which maintains the lookup table as in the previousembodiment is not required here. Similar to the previous embodiment, thecalibration factor is transmitted to the sensors 310A-310D either byone-sensor-at-a-time or multiple-sensors-at-a-time methods. In thevarious embodiments disclosed herein, the reference instrument includesa beta attenuation monitor (BAM), which is commercially available fromsuch manufacturers as Thermo Fisher Scientific, Inc. of Minneapolis,Minn., or includes one or more low cost sensors such as described hereinin FIGS. 1-3. In this example embodiment, a calibration factor isdetermined or generated by comparing the low cost sensor reading withother low cost sensors in the same general area.

Referring now to FIG. 7, there is illustrated a system 400 and method ofcalibrating particle sensors 410A-410C that does not use an internetserver and a transfer standard in accordance with an example embodimentof the invention. In this embodiment of the invention, a calibrationfactor is determined or generated without the transfer standard bycomparing the data from the reference instrument 420 and one particlesensor, such as sensor 410A. Unlike the previous embodiments, thiscalibration scheme or configuration uses two-way communication 470 toaccomplish the remote sensor calibration against a standard or referenceinstrument. When calibrating multiple sensors, control device 480communicates with reference instrument 420 and with one or more sensors410B-410C to arrive at a calibration factor and ultimately calibratingthe remote particle sensors.

One application of the remote calibrating method and systems describedherein are for calibrating aerosol optical sensors deployed in thefield. Optical sensors such as optical particle counters, photometersare commonly used in the aerosol monitoring field. These sensors eithermeasure the light scattered or attenuated by the particles. The lightsignal depends strongly on the aerosol properties namely refractiveindices and morphology. The effect of the refractive indices andmorphology can be taken into account by either performing theoreticalscattering modeling if aerosol optical properties are known orcalibrating the instruments against a measurement reference.

If the optical sensors are calibrated against a non-optical referenceinstrument, there is an additional benefit that optical signals measuredby these sensors would automatically be converted to another aerosolproperty of interest. For instance, if the optical sensors arecalibrated against a reference mass measurement instrument, instead ofshowing optical signals, these sensors can provide aerosol massinformation. If the reference instrument is an aerodynamic sizemeasurement instrument, these optical sensors can then be used tomeasure aerosol aerodynamic size after calibration. Ideally, opticalsensors should be re-calibrated every time the composition of theaerosol changes. In practice, however, calibration is usually only doneinfrequently because: 1) the reference instruments are typically onlyavailable in the laboratories or certain fixed locations due to theirlarge size and high cost, or 2) the calibration process is typicallylabor intensive and expensive hence it is not practical to performcalibrations frequently in the field. The teachings herein provide amethod to calibrate these optical sensors easily and without therequirement to have a reference instrument located in the same place. Inthis example, optical sensors are calibrated to monitor mass ofparticulate matter (PM) less than 2.5 μm. This mass value is commonlyreferred to as PM2.5 and it is widely used as an air quality indicator.

The reference instrument in this example embodiment is a betaattenuation monitoring (BAM) instrument. The BAM determines the mass ofparticulate matters by comparing the beta radiation attenuation beforeand after the sample is collected on a filter or thin film. The BAM iscommonly used by United States Environmental Protection Agency (EPA) tomonitor PM2.5 at various monitoring sites. The technique is widely usedin other countries as well. The mass information collected by the BAMdevices is usually available to the public on the air monitoringagencies web sites. Some monitoring sites even send the information outhourly using the social media service Twitter. The transfer standardmodule used in this example embodiment is an optical particle counter orphotometer located in a weatherproof enclosure to protect it from theelements. Several of these transfer standard modules could be deployedin big cities where BAM information is available and the data collectedby these transfer standard modules is sent to a cloud server. Bycomparing the data/information from the BAMs and transfer standardmodules, calibration factors at various locations/cities can begenerated and a lookup table is generated and uploaded to a web serveror other network storage location. By using a mobile device, the lookuptable can be pulled from the server and the appropriate calibrationfactor could be transmitted to one or more particulate matter sensorslocated indoors or outdoors. Using the methods described herein, thevarious particulate matter sensors in a specified city could becalibrated daily against a reference instrument (in this case a BAM or alow cost sensor as described herein) located somewhere in the city. Thedaily calibration ensures these sensors take into account the type ofaerosol present in that city/region at that time, and thus provide auseful and credible reading.

The following patents and publications are incorporated by reference intheir entireties: U.S. Pat. Nos. 5,121,988; 7,111,496 (BAM devices);U.S. Pat. Nos. 7,932,490; 8,009,290; and 8,351,035 (sensor calibration).

The foregoing specific embodiments of the present invention as set forthin the specification herein are for illustrative purposes only. Variousdeviations and modifications may be made within the spirit and scope ofthe invention without departing from the main theme thereof.

What is claimed is:
 1. A sensor assembly for sensing low concentrationsof particulate matter comprising: a housing having an air channelconfigured to direct a sampled particle aerosol through the housing; aparticle sensor device having a light emitting unit and a lightreceiving unit each directed towards a flow channel, wherein the flowchannel is configured to receive at least a portion of the sampledparticle aerosol; and a blower member disposed adjacent the air channelof the housing and the flow channel of the particle sensor device, theblower member configured to periodically push air through the flowchannel of the particle sensor device; wherein the blower member is oneof a microblower member or a micropump.
 2. The sensor assembly of claim1, further including a filter element disposed adjacent the blower andthe air channel of the housing, the filter element configured to filterparticulates from an air flow.
 3. An air quality monitoring systemcomprising the sensor assembly of claim 1 communicatively coupled to atleast one of a mobile device or a server.
 4. The system of claim 3further comprising a plurality of sensor assemblies configured to bepart of a wireless network.
 5. The system of claim 4, wherein the sensorassemblies are configured to transmit out particle data to the at leastone of the mobile device or the server.
 6. The system of claim 3,further comprising a plurality of sensor assemblies configured to bepart of a wired network.
 7. The system of claim 3, wherein the sensorassembly is configured to transmit out particle data to at least one ofthe mobile device or the server.